Practice Problems Week 5 2019

Practice Problems Week 5 1. Irregular field is defined as a field: a. Other than a square or a circle b. Other than a square, a circle, or a rectangle c. Smaller than 4 x 4cm d. Larger than 40 x 40cm e. Other than a rectangle 2. The Clarkson method of dose calculation for irregular fields is performed using the following set of data: a. TAR(0) and SAR b. %DD and BSF c. TAR(0) and %DD d. TMR(0) and %DD e. TMR and BSF 3. The calculation of dose at a point away from the central axis required the knowledge of the: a. Off-axis ratio b. Tissue-phantom ratio c. Photon fluence d. Photon flux e. Kerma 4. If a field is 10cm long at 100SSD, what is its length at an extended distance of 150cm SSD? 5. Given a magnification factor of 1.28 and a physical measurement of 18 x 13cm on the patient, what should the field measure on the film? 6. Two fields are planned to abut at a depth of 6cm. Calculate the skin separation (gap) for 20 x 20 and 10 x 10 cm fields using 100cm SSD. 7. Collimator scatter is measured using a. The dose in air for different collimator jaw settings b. The dose in phantom at dmax depth for different collimator jaw settings c. The dose in phantom at 5cm depth for different collimator jaw settings d. The dose in air with the collimator jaw closed

e. The dose in phantom at dmax depth with the collimator jaw open for the largest field possible 8. The wedge transmission factor is .590. The MU setting for the beam without the wedge is 150 mu. What is the mu setting for the same dose with the wedge? 9. When treating a patient with 2 60º oblique ports, 120º apart, the optimum wedge angle is: 10.. What is the A/P for 5 x 30cm field? a. 8.57 b. 0.50 c. 4.29 d. 2.14 11.What is the equivalent square for a 10 x 25cm field? a. 3.57 b. 7.14 c. 17.5 d. 14.3 12.The thickness of lead required to shield a 6 MeV electron beam is approximately ________mm. a. 0.5 b. 3 c. 6 d. 12 e. 24 13.The minimum distance one should keep trays and blocks away from the patient’s skin surface for a photon beam to preserve the skin sparing effect is: a. 10 cm b. 15 cm c. 35 cm d. 5 cm

14.A 4 HVL block reduces the beam intensity to ________%.

15.For an isocentric treatment set-up, calculate the effective TAR for a 10 x 10cm 60C0 beam at a depth of 10cm using the data and tables provided below. TAR0 = .534 a. 0.534 b. 0.361 c. 0.173 d. 0.707 e. 0.389 Radius 1 2 3 4 5 6 7 8 9

Length (cm) 5.0 5.1 5.3 5.8 6.6 6.6 5.8 5.3 5.1

SAR 0.161 0.163 0.167 0.176 0.192 0.192 0.176 0.167 0.163